Having upgraded from a pop-up to our TM we know exactly the issue you are referring to.
In the pop-up I ended up lining the plywood base under the mattress with a thin layer of insulating 'underlayment' left over from a hardwood flooring project, which helped. However, we have camped in colder weather with the TM and so far have felt no need to make any modifications to it.
While there is some cold working through as is inevitable when there is nothing but air underneath you, the platform under the bed in a TM is actually filled with a foam core insulator, unlike the plain plywood base in a pop-up. The difference in insulating properties is substantial.
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